Remove visibiliytchange event listeners when no longer required (#627)

* Remove visibiliytchange event listeners when no longer required

* Update comment

* Wording cleanup

Author: tunetheweb

src/lib/whenIdleOrHidden.ts

@@ -29,7 +29,12 @@ export const whenIdleOrHidden = (cb: () => void) => {
     cb();
   } else {
     cb = runOnce(cb);
-    rIC(cb);
     document.addEventListener('visibilitychange', cb, {once: true});
+    rIC(() => {
+      cb();
+      // Remove the above event listener since no longer required.
+      // See: https://github.com/GoogleChrome/web-vitals/issues/622
+      document.removeEventListener('visibilitychange', cb);
+    });
   }
 };